#13906: REPEATABLE READ (as used by default on MySQL) breaks atleast
QuerySet.get_or_create().
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Reporter: | Owner:
sebastian_noack | Status: new
Type: | Component: Database layer
Cleanup/optimization | (models, ORM)
Milestone: 1.4 | Severity: Normal
Version: SVN | Keywords: mysql transaction
Resolution: | isolation
Triage Stage: Accepted | Has patch: 1
Needs documentation: 1 | Needs tests: 0
Patch needs improvement: 1 | Easy pickings: 0
UI/UX: 0 |
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Comment (by akaariai):
According to my quick test, this only works if you have done no queries at
all before taking the savepoint. No matter which table you select from
before the SAVEPOINT p1. So, this would be only a minor improvement:
get_or_create works correctly, but only if it is the first operation in
the transaction. You can see this if you put any select above the SNAPSHOT
P1; for process 1.
The reason seems to be that MySQL takes the snapshot only when first
needed. But if it happens to take the snapshot inside savepoint, for some
strange reason it rolls the snapshot back when rollback to savepoint is
called, and creates a new snapshot for the second select. If anything,
that is a bug in MySQL, because you just had a non-repeatable read in
repeatable read isolation.
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Ticket URL: <https://code.djangoproject.com/ticket/13906#comment:30>
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